The present invention relates to a high-speed radial tire for heavy load which improves the critical speed of generating standing waves and the durability of a bead part.
Since the size and the flight speed of aircraft have been increased, the operating speed and the acting load of tires have been increased, and therefrom, a larger durability is required in high-speed radial tires for heavy load, especially in tires for aircraft, for a safe take-off and landing.
A tire for aircraft must meet various characteristics as mentioned below as well as tire durability, including every part of the tire such as wear resistance, resistance to rolling, low heat generating characteristics and the like.
1. To possess a resistance against a succession of large deformations, since the deformation quantity of the tire is significantly as large as 28 to 38%, for example, in a loaded state, in order to effectively lower the shock when an aircraft takes off or lands on a lading strip. PA0 2. To possess a resistance against a high-speed rotation in a heavy load and a large deformation, since the take-off and landing speed has been increased due to the flight ability of aircraft at higher speed. PA0 3. To be able to resist taxiing conditions when moving between the landing strip and the gate, where a large load acts for a relatively long time in spite of lower speed. PA0 4. To posses resistance against a load of approximately 130 to 360 times (approximately 50 times in normal tire) per unit weight of tire due to lightening of the aircraft, and a significantly high internal pressure such as 10 to 16 kg/cm.sup.2 created thereby.
On the other hand, as such a tire for aircraft, having a cross-ply construction in which carcass cords are placed to mutually cross between the plies are widely used. However, due to their smaller rigidity in tread part and heavier weight as well as wear resistance and heat generating characteristics, such tires are not preferable, and those having a cross-ply construction are limited in their use because of a remarkably improved performance of recent large-sized jet-propelled aircraft.
Therefore, recently, radial tires having a belt layer composed of belt cords with a high elasticity inclined at a small angle against the equator of the tire placed outside in the radial direction of the carcass in so-called radial or semi-radial construction where carcass cords are placed in a row in the radial direction of the tire are gradually used.
However, it has been found that such tires for aircraft with a radial construction have a relatively lower durability in the bead part compared with the durability of the entire tire. It has also been found that standing waves tend to be generated at a relatively high rate.
In regard to the durability in the bead part, since deforming quantity in the radial direction of the tire upon loading is as large as approximately 28 to 38%, as mentioned before, compressive stress acts on the carcass cords of the carcass A in the bead part, and in the folded part about the bead core B, as shown in FIG. 8, and it is evident that in the inside, in the direction of tire's axis, a tension which tenses the carcass cords is generated.
On the other hand, it has been found that damage to the bead part occurs near the top end of the rim flange C. From the results of a drum test, it was found that as the bead part bends to the top edge D in the top end of the rim flange C extending outward to the tire's axis, a compression is created by the rapid bending in this part, which generates a centralized compressive stress such that the part is broken by fatigue accompanied by a succession of compressive strains caused by the large compressive stress which acts on the carcass cords of the part, and that as the broken end promotes the centralization of compressive stress in a specific part, damage to the bead part is generated. This bending increases the contact pressure of the bead part in the top edge D, and as the contact pressure increases, the compressive stress and the damage increases.
Moreover, the standing wave is a wave phenomenon in the tread part of a tire while driving, and such waves in the tread part cause a deterioration in the tire's durability, especially by exciting the bead part through the side-wall part of the tire.
In a heavy-loading, high-speed radial tire for aircraft, as mentioned before, since the deformation is significant and the take-off and landing speed is high, exceeding 300 km/h, such standing waves tend to be generated. As generally known, standing waves in a radial tire can be obtained from the following formula (1). ##EQU1## where Vc: critical speed of standing wave generation;
m: mass of unit length in the tread part; PA1 EI: bending rigidity in the tire's surface, in the tread part; PA1 T: tension of belt; and PA1 k: spring constant of carcass.
The formula (1) was obtained by supposing the belt layer as an infinite beam supported elastically by the carcass, and it is known that in order to improve the critical speed Vc of generating standing waves, the mass m should be lowered and the rigidity EI, tension of belt T and spring constant of the carcass k should be increased.
Meanwhile, in order to increase the specified frequency of the belt and to improve the critical speed Vc of generating standing waves without decreasing the mass m, it was proven that a larger tension T should be applied to the belt by filling the tire with an internal pressure, and thereby the apparent bending rigidity EI in tire's surface in tread part is increased and the critical speed Vc is improved.
In addition, while in the formula (1), it is supposed that an equal tension T acts on the belt layer, it was also found by experiments that by applying a larger expansion especially in the equatorial part of a tire, that is, the crown part of the tread part, it is effective to improve the critical speed of the standing waves and that the tension of the belt T in this part is relatively increased.
For this purpose, the radius of the inner circumference of the tread part, especially that of the belt layer is preferably set at a smaller length and the belt camber quantity at a larger quantity compared with conventional tires. In the case that the radius of the side-wall part is mainly set approximately at the same length as in conventional tires, this helps to lower the contact pressure with the flange of the bead part, and it is also proves that this improves the durability of the bead part.
Moreover, in addition to the above, it was found that an increase in expansion in the crown part helps to standardize the distribution of ground contact pressure in the tread surface.